I did try later a half GOMAD, seemed to work pretty good.

Yeah that’s one of the possible problems with GOMAD 🙂
This strategy is not for everyone. Some people respond better than others. Some questions you can ask yourself, are you lactose intolerant? Sometimes people don’t notice that they are, since they only drink low amounts of milk, but with the amount of milk needed for GOMAD lactose intolerance can really manifest itself.
I guess one way to lessen the amount of crapping could be easing into GOMAD, starting off slowly and then periodically increasing the amount of milk you drink, that way your body becomes accustomed to it. You can also spread out the amounts of milk you drink throughout the day. Instead of drinking big amounts in one sitting, try to spread out smaller amounts of milk throughout the day.
This might work and it might not. GOMAD is a strategy that can create unwanted side-effects with certain people and so only works for certain people and for others it doesn’t.